Add a "needs review" flag with a sidebar filter

Links can be bookmarked for a later look via a toggle button on the
card (🏷️/🔖), shown with a yellow border while flagged. A new
sidebar section "Merkliste" links to /?review=1, filtering to just
the flagged links; the count updates live via the existing facets
query.

Since the project has no migration framework, add a minimal
startup check that ALTER TABLEs in the needs_review column for
existing SQLite databases (create_all only creates missing tables).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/base.html

@@ -57,6 +57,8 @@
     #links > #empty-state { grid-column: 1 / -1; }
 
     .link-card { display: flex; flex-direction: column; }
+    .link-card.needs-review { border-color: #ca8a04; }
+    .review-toggle.active { color: #fbbf24; border-color: #ca8a04; }
     .card-head { display: flex; align-items: flex-start; gap: 8px; margin-bottom: 10px; }
     .card-avatar {
       flex-shrink: 0; width: 34px; height: 34px; border-radius: 9px;